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The phrases circulatory system and cardiovascular system are often used interchangeably to describe the bodily system that pumps blood from the heart throughout the body. While this is the general function of the cardiovascular system, it is only one function of the circulatory system. The circulatory system is that which circulates not only blood, but lymph, hormones, and nutrients among the body’s tissues, and in turn eliminates the byproducts of cellular metabolism. Therefore, the cardiovascular system and lymphatic system are subdivisions of the circulatory system, which includes the heart, blood vessels like arteries and veins, and the blood itself, as well as the lymph vessels, lymph nodes, and the watery fluid known as lymph.
In some respects, the terms circulatory system and cardiovascular system may be used synonymously. When discussing the circulation of blood and the nutrients it contains, both are applicable. The purpose of blood circulation is to distribute oxygen, glucose, electrolytes like sodium and potassium, and other nutrients to the organs, muscles, and other bodily tissues via the arteries and capillaries. At the same time, blood that has been depleted of these nutrients is returned via capillaries to the veins and cycled back to the heart. The heart will then pump this blood into the lungs to dispose of carbon dioxide, a major metabolic byproduct, and to receive a fresh supply of oxygen.
These are the primary functions of the circulatory system and the reason that circulatory system and cardiovascular system tend to mean the same thing, while the lymphatic system tends to be differentiated from the cardiovascular system. This may be because the cardiovascular system can be said to be a closed loop system, meaning that the blood circulated within is confined to the the heart and blood vessels. It therefore cycles continuously through the system with no point of entry or exit.
The lymphatic system, by contrast, is an open system, meaning that it is made up of a network of lymph vessels and nodes, or organs, that drain lymph out of the system and into the bloodstream. Its function is to eliminate waste products that build up in the body’s tissues, from pathogens and toxins to water and other excess fluids. The lymphatic system also contributes to cardiovascular system function. Specifically, it transfers the hormones, nutrients, and oxygen found in blood to the tissues that require it while draining the waste produced during the metabolism of these substances back into the bloodstream. For this, the lymphatic system is often considered a division of the circulatory system.